Precoding of multi-antenna transmissions is an increasingly popular technique used in several advanced wireless communications standards. Precoding techniques include single-layer beamforming, where the same signal is emitted from each of several transmit antennas, but with different precoding weights applied to each of the antennas so that the signal power is maximized at the receiver output. When the receiver has multiple antennas, precoding is used for multi-layer beamforming in order to maximize the throughput performance of a multiple receive antenna system. With multi-layer precoding, multiple data streams are simultaneously transmitted, with independent weights applied to each antenna to maximize the link capacity or quality.
The Wideband Code-Division Multiple Access (W-CDMA) and Long-Term Evolution (LTE) standards promulgated by the 3rd-Generation Partnership Project (3GPP) each permit linear precoding on the downlink, when multiple transmit antennas are used to serve a mobile station (User Equipment, or “UE”, in 3GPP parlance). The transmitter antenna precoding vector that maximizes the data rate received by each mobile depends on the instantaneous downlink channel (including fading) to the mobile; hence, implementing this optimal precoder requires the network to acquire channel state information characterizing the propagation conditions between each of the transmit antennas and the mobile station. In an Orthogonal Frequency-Division Multiple Access (OFDMA) system like LTE, this channel state information must be acquired for each pair of receive/transmit antennas (i.e., M×N single-input/single-output channels, where M is the number of transmit antennas and N is the number of receive antennas).
Codebook-based linear precoding is a technique that reduces the amount of information needed to feedback from the mobile to the network for implementing the linear precoding. With a traditional codebook-based precoding, a fixed set of allowed precoder weighting vectors (a codebook) is chosen a priori, and each precoding vector in this set is assigned a unique index that is known to both the network and the mobile stations. Each mobile station measures its downlink channel (of size M×N) and determines the “best” precoding vector belonging to the codebook, given the downlink channel measurements. The mobile station feeds back an index of this best precoding vector to the network, so that subsequent transmissions from the network can be precoded according to the selected vector.
Assuming a codebook consisting of L precoding vectors, log2(L) bits are needed to uniquely identify a single precoding vector. Typically, log2(L) bits is fewer than the number of bits needed to characterize the M×N downlink channel each mobile station sees; thus, the codebook-based approach reduces the amount of signaling information that must be transmitted between the mobile station and the network.
In WCDMA and LTE systems, a multi-antenna transmission to a mobile station is generally transmitted from only a single point, i.e., a single transmitter site. In other words, the M transmit antennas used to serve a given mobile are usually co-located. In this case, the channel between every one of these antennas and the mobile station has the same path loss and shadowing. The codebooks in LTE and WCDMA are designed specifically for the case when all the transmit antennas are co-located, i.e., for the case in which the path loss between each of the transmitter antennas and the mobile station is the same, or very close to the same. (The term “path loss” is sometimes used to refer only to the propagation loss caused by distance between the transmitter and receiver, whether a free-space model, i.e., where path loss is proportional to the distance squared, or an empirically derived model, e.g., where path loss is proportional to the distance raised to the fourth power, is used. “Shadowing,” on the other hand, generally refers to losses caused by particular features of the environment in a given scenario, such as losses caused by the proximity of a large building or geological feature. In the remainder of this disclosure, however, the term “path loss” is generally intended to include both of these phenomena, unless the context indicates otherwise, but to exclude the distinct phenomena of fading, which results from the destructive combining of multipath components of the transmitted signal at the receiver.)
In developing the specifications for the so-called LTE-Advanced system, 3GPP members are considering the use of true multi-point transmission (i.e., from multiple transmitter sites), where the M transmit antennas used to serve a given mobile station could be located at several different geographical locations. In these scenarios, the channels between the mobile station and various ones of these antennas could have different path losses. Conventional codebook-based techniques for specifying a particular precoding vector to be used, given rapidly varying channel conditions, are poorly suited for the multi-point transmission scenario.